Law enforcement officials said 20 children — ages 5 to 10 — and six adults are among the dead at the school.

The gunman shot and killed himself inside the school. The shooter’s mother, Nancy Lanza, was found dead at their home in Newtown, five miles away from the school.

WBZ-TV’s Christina Hager reports from Newtown

Nancy Lanza was originally reported to be a teacher at Sandy Hook Elementary.

However, law enforcement officials now say Lanza was not employed at the school as a teacher, substitute teacher or aide. Investigators are trying to establish whether there was a link to the school.

Officials say most of the shooting took place in two classrooms.

A source said that the shooter was wearing dark clothing during the attack and apparently knew his way around the school. It is unclear how he gained access.

The shooter’s older brother, Ryan, was being questioned by police in New Jersey. There is no indication that he was involved in the shooting at this point. The shooter’s father, Peter Lanza, lives in Stamford, Connecticut and is a vice president of taxes for GE Energy Financial Services.

WBZ-TV’s Karen Anderson reports

State Police say Newtown police officers received the first 911 call just after 9:30 a.m Friday.

They requested State Police assistance as soon as the call came in. All on and off-duty troopers were called to respond to the scene. They swarmed the school immediately when the arrived.

Robert Licata said his 6-year-old son was in class when the gunman burst in and shot the teacher.

“That’s when my son grabbed a bunch of his friends and ran out the door,” he said. “He was very brave. He waited for his friends.”

He said the shooter didn’t utter a word.

Several guns, including two pistols, a Sig and a Glock, were found in the school and a Bushmaster rifle was found in a car. At least three of the guns were purchased legally, and registered to Adam’s mother.

The principal has been identified as one of the victims.

Dawn Hochsprung was a well-liked and experienced administrator, and had been principal at Sandy Hook since 2010. Hochsprung was a principal in Woodbury schools before taking the job in Newtown.

Another adult victim has been identified as Mary Sherlock, the school psychiatrist, who was set to retire at the end of the year.

Photos from the scene showed young students — some crying, others looking visibly frightened — being escorted by adults through a parking lot in a line, hands on each other’s shoulders.

Stephen Delgiadice said his 8-year-old daughter was in the school and heard two big bangs. Teachers told her to get in a corner, he said.

“It’s alarming, especially in Newtown, Connecticut, which we always thought was the safest place in America,” he said. His daughter was fine.

Andrea Rynn, a spokeswoman at the hospital, said it had three patients from the school but she did not have information on the extent or nature of their injuries.

Mergim Bajraliu, 17, heard the gunshots echo from his home and ran to check on his 9-year-old sister at the school. He said his sister, who was fine, heard a scream come over the intercom at one point. He said teachers were shaking and crying as they came out of the building.

“Everyone was just traumatized,” he said.

Richard Wilford’s 7-year-old son, Richie, is in the second grade at the school. His son told him that he heard a noise that “sounded like what he described as cans falling.”

The boy told him a teacher went out to check on the noise, came back in, locked the door and had the kids huddle up in the corner until police arrived.

“There’s no words,” Wilford said. “It’s sheer terror, a sense of imminent danger, to get to your child and be there to protect him.”

Connecticut Governor Daniel Malloy was on the scene in Newtown. Malloy called the shooting “a tragedy of unspeakable terms” and asked the world to remember the victims and to pray for them.

The nation’s flags have been ordered at half-staff through Tuesday.

An emotional President Barack Obama held a news conference at the White House Friday afternoon, calling for the nation to come together. The president wiped away tears as he mourned the victims and promised to “take meaningful action.”

President Obama paused several time as he struggled to keep his composure while speaking of the children who had died and the life milestones they now would miss.